There are several variations but below are two of the most commonly used replies.
In back-and-forth dialog, the conversation would go like this.
Hay is for horses
Straw is cheaper
Grass is free
Buy a farm and you get all three!
Hay is for horses,
Straw is for cows,
Milk is for little pigs
Swill for the sows
You can check other suggestions at Alphadictionary.com.
The English idiomatic phrase “hay is for horses” is a specific idiom with a particular meaning and use; at its core, the phrase is really a remonstration for someone using the word “hey” as a verbal opener, to address someone or start a sentence. The idiom uses a pun on the words “hey” and “hay,” which are “homophones” or words that sound alike. If someone says “hey,” someone else might answer “hay is for horses.”
The implicit second part of this phrase is that since hay is for horses, it is not for people. This kind of idiom was popular in older times when more formal uses of English prevailed in many English-speaking societies. These days, it has become less common to address someone’s use of the word “hey,” although this still may not be appropriate in a more formal setting.
Although people might not use this phrase that much anymore, it remains a primary example of an idiom that has a more focused purpose and use. Many idioms are just describing situations, but this phrase is actually a somewhat veiled imperative that many might call “passive aggressive,” because it avoids actually telling someone not to say the word “hey,” but instead, suggests the change in a roundabout way. - See more at Wisegeek.com.